Cushnie studied at Greenmount College of Agriculture & Horticulture, Antrim.
He specialised in glasshouse crops and fruit growing and spent six years with the Northern Ireland Horticultural Advisory Service, then seven years controlling landscape maintenance with the Northern Ireland Housing Executive.
In 1978 he started his own landscape contracting business, Cushnie Landscapes, working on private gardens and for local authorities.
His broadcasting career started at BBC Radio Ulster. He was a regular panellist on Gardeners' Question Time for 15 years and a prolific journalist and author. There are no immediate plans to replace him on the show.
The tribute edition will air on Radio 4 at 3pm and will be repeated on 17 January at 2pm.
Two more highly respected figures have also died, including Brogdale trialist Hugh Ermen, who passed away aged 81.
Ermen trained at Writtle and Wisley and was a member of the National Fruit Trials team at Brogdale for 29 years, under directors including Jock Potter.
He developed a consummate knowledge of fruit varieties, pollination and tree-growing techniques born out of a canny ability to observe and note tree-growing habits.
He catalogued the Brogdale collections and contributed photographs of them to John Bultitude's book Apples.
As an amateur plant breeder in retirement, his legacy will be a stable of new apple varieties including Red Devil, Herefordshire Russet and Scrumptious.
Ermen investigated the propagation of cultivars using the Malling Bin technique and he was awarded the RHS Associate of Honour in 1988 for his contributions to pomology.
A gentle horticulturist with a quiet humour, he leaves his widow, Laura, after whom he named a fastigiate ornamental crab, Malus 'Laura'.
The fruit world is also mourning Canterbury fruit grower Hugh Kelsey, who has passed away, aged 79.
Kelsey farmed at Wickhambreaux, near Canterbury, for all of his life. In the 1950s he took over his family's mixed farming business, rearing cattle and sheep as well as growing a range of fruit crops.
However, it was the production of apples and pears to which he was ultimately drawn.
His fruit production was highly acclaimed within the fruit co-operatives, Kentish Garden Growers and East Kent Packers (EKP). He served as a director on both boards.
Former fellow EKP member and East Kent Fruit Society president David McEwen said: "Hugh was such an enthusiast, whether playing tennis, sailing or riding his horses, or indeed driving his sports cars in his younger days."
He added: "Hugh's endearing legacy will be one of courteousness, generosity and kind-heartedness — and, of course, his fruit growing was impeccable".
In 2001, Kelsey was honoured to serve as master of the Worshipful Company of Fruiterers.
Kelsey is survived by his wife, Caroline, and two sons — one of whom, Paul, continues in the fruit-growing business — and six grandchildren.